Modified preamble structure for IEEE 802.11a extensions to allow for coexistence and interoperability between 802.11a devices and higher data rate, MIMO or otherwise extended devices

ABSTRACT

A modified preamble is used by extended devices that operate at higher rates, MIMO or other extensions relative to strict 802.11a-compliant devices. The extended devices might use multiple antenna techniques (MIMO), where multiple data streams are multiplexed spatially and/or multi-channel techniques, where an extended transmitter transmits using more than one 802.11a channel at a time. Such extensions to IEEE 802.11a can exist in extended devices. The modified preamble is usable for signaling, to legacy devices as well as extended devices, to indicate capabilities and to cause legacy devices or extended devices to defer to other devices such that the common communication channel is not subject to unnecessary interference. The modified preamble. is also usable for obtaining MIMO channel estimates and/or multi-channel estimates. The modified preamble preferably includes properties that facilitate detection of conventional and/or extended modes (“mode detection”) and provides some level of coexistence with legacy IEEE 802.11a devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from co-pending U.S. Provisional PatentApplication No. 60/461, 999, filed Apr. 9, 2003 entitled “MODIFIEDPREAMBLE STRUCTURE FOR IEEE 802.11A EXTENSIONS,” which is herebyincorporated by reference, as if set forth in full in this document, forall purposes.

BACKGROUND OF THE INVENTION

The IEEE 802.11a standard defines data rates of 6 Mbps (megabits persecond) up to 54 Mbps. For some applications, higher data rates forgiven modulations and data rates higher than 54 Mbps are desirable.Other extensions, such as the use of MIMO (multiple-input,multiple-output antenna systems and other extensions might be desirable.In order to avoid conflicts with existing standardized communicationsand devices, extended devices that extend beyond the limits of the802.11a standard and legacy devices that comply with the existingstandard and are not necessarily aware of extended standards both needto coexist in a common communication space and even interoperate attimes.

Coexistence is where differing devices can operate in a common space andstill perform most of their functions. For example, an extendedtransmitter transmitting to an extended receiver might coexist with alegacy transmitter transmitting to a legacy receiver and the extendeddevices can communicate while the legacy devices communicate, or atleast where the two domains are such that one defers to the other whenthe other is communicating. Coexistence is important so that theadoption and/or use of extended devices (i.e., devices that are outside,beyond or noncompliant with one or more standards with which legacydevices adhere and expect other devices to adhere) do not requirereplacement or disabling of existing infrastructures of legacy devices.

Interoperability is where an extended device and a legacy device cancommunicate. For example, an extended transmitter might initiate atransmission in such a manner that a legacy device can receive the datasent by the extended transmitter and/or indicate that it is a legacydevice so that the extended transmitter can adjust its operationsaccordingly. For example, the extended transmitter might revert tostandards compliant communications or switch to a mode that, while notfully standards compliant, is available to the legacy receiver. Inanother situation, an extended receiver might successfully receive datafrom a legacy transmitter.

The IEEE 802.11a standard defines a 20 microsecond long preamble with astructure as shown in FIG. 1, having short training symbols S (0.8microseconds each), a guard interval LG, long training symbols L (3.2microseconds each) and a signal field (4 microseconds). The preamble isfollowed by data. The first eight microseconds comprises ten identicalshort training symbols that are used for packet detection, automaticgain control and coarse frequency estimation. The second eightmicroseconds comprise two identical long training symbols, L, precededby a guard interval LG that is the same pattern as the last half (1.6microseconds) of the long training symbol L. The long training symbolscan be used for channel estimation, timing, and fine frequencyestimation.

FIG. 2 shows a long training sequence, L₁, that is used to generate thesignal representing the long training symbol in a conventional 802.11apreamble. This sequence represents values used over a plurality ofsubcarriers. As specified in the standard, the subcarriers span a 20 MHzchannel and with 64 subcarriers, they are spaced apart by 312.5 kHz. Byconvention, used here, the first value in the sequence is the value forthe DC subcarrier, followed by the value for the 1×312.5 kHz subcarrier,then the value for the 2×312.5=625 kHz subcarrier, etc., up to the 32ndvalue for the 31×312.5 kHz=9687.5 kHz subcarrier. The 33rd valuecorresponds to the −10 MHz subcarrier, followed by the −(10 MHz −312.5kHz) subcarrier, and so on, with the 64 value being for the −312.5 kHzsubcarrier.

As can be seen from FIG. 1, the DC value and the 28th through 38thvalues, corresponding to the edges of the 20 MHz channel, are zero. Theoutput of a transmitter is a training symbol at a sample rate of 64samples/symbol. The samples are obtained by taking a 64-point IFFT(inverse fast-Fourier transform) of the long training sequence, L₁ inthis example. As used herein, a sequence in the frequency domain isexpressed with uppercase letters (e.g., L(k)), while the correspondingtime sequence is expressed with lowercase letters (e.g., l(k)).

One approach to obtaining higher data rates is the use of morebandwidth. Another approach, used by itself or as well as the use ofmore bandwidth, is MIMO (multiple-input, multiple-output) channels,where a plurality of transmitters transmit different data or the samedata separated by space to result in possibly different multi-pathreflection characteristics. In either case, care is needed forcoexistence and interoperability between legacy devices and extendeddevices.

BRIEF SUMMARY OF THE INVENTION

A modified preamble is used by extended devices that operate at higherrates, MIMO or other extensions relative to strict 802.11a-compliantdevices. The extended devices might use one or more of multiple antennatechniques (MIMO), where multiple data streams are multiplexed spatiallyand multi-channel techniques, where an extended transmitter transmitsusing more than one 802.11a channel at a time. Such extensions to IEEE802.11a can exist in extended devices.

The modified preamble is usable for signaling, to legacy devices as wellas extended devices, to indicate capabilities and to cause legacydevices or extended devices to defer to other devices such that thecommon communication channel is not subject to unnecessary interference.The modified preamble is also usable for obtaining MIMO channelestimates and/or multi-channel estimates.

The modified preamble preferably includes properties that facilitatedetection of conventional and/or extended modes (“mode detection”) andprovides some level of coexistence with legacy IEEE 802.11a devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the structure of a conventional 802.11a preamble.

FIG. 2 shows a long training symbol sequence, L₁, used for aconventional 802.11a preamble.

FIG. 3 illustrates several devices coupled via a wireless network.

FIG. 4 illustrates other long training sequences, usable by extendeddevices.

FIG. 5 illustrates one possible layout for out-of-band pilot tones forindividual channels.

FIG. 6 illustrates one possible layout for out-of-band pilot tones forcommonly assigned adjacent individual channels, where the out-of-bandsignals between adjacent bands are not attenuated.

FIG. 7 illustrates a layout for out-of-band pilot tones for fouradjacent individual channels assigned to a single device, where theout-of-band signals between adjacent bands are not attenuated.

FIG. 8 illustrates a modified preamble usable for multi-channel packetswith or without MIMO.

FIG. 9 is a flowchart illustrating one possible process for obtainingchannel estimates for each transmitter signal in a MIMO system.

DETAILED DESCRIPTION OF THE INVENTION

The use of modified preambles is described herein. Such modifiedpreambles can be used in packets sent over a wireless network, such asan 802.11a compliant wireless network. Such packets with modifiedpreambles can be sent by transmitters according to embodiments of thepresent invention to be received by receivers according to embodimentsof the present invention, as well as being received by legacy receiversthat are not configured to receive and interpret the modified preamblesas would be done with receivers according to embodiments of the presentinvention.

FIG. 3 illustrates just one example of a wireless network being used forcommunications among transmitters and receivers as indicated. As shown,two wireless devices 102(1), 102(2) might use and interpret the modifiedpreambles, while a legacy wireless device 104 might not be expecting themodified preambles, but might hear signals representing such preambles.Extended wireless devices 102 might operate using multiple channelsand/or multiple transmit antennas and/or multiple receive antennas.Devices might have a single transmit antenna and a single receiveantenna, or more than one transmit antenna and/or more than one receiveantenna. While separate transmit and receive antennas are shown,antennas might be used for both transmitting and receiving in somedevices.

Border 106 is not a physical border, but is shown to represent a spacewithin which signals can be received from devices within the space.Thus, as one device transmits a signal representing a packet withinborder 106, other devices within border 106 pick up the signals and, asthey are programmed, will attempt to determine if the signals representpackets and if so, then demodulate/decode the packets to obtain the datarepresented therein.

Many variations of a modified preamble might be used. An example is thepreamble shown in FIG. 1, where the long training symbol is modified touse sequences such as one of the example sequences shown in FIG. 4.

Preferably, a modified preamble will be such that 1) an extendedreceiver (e.g., one that can advantageously handle modified preambles)can distinguish between MIMO packets (or other extended mode packets)and conventional 802.11a packets, 2) a legacy receiver (e.g., one thatis not configured to receive and interpret the modified preambles andmight not expect extended operations) can receive enough of a packet todetermine either that the legacy receiver can understand the packet orcan defer processing of incoming signals for a time, thereby allowing ameasure of coexistence, 3) the modified preamble is usable for MIMOsynchronization and channel estimation, and 4) the modified preamble isuseful in a process of detecting the use of multi-channel transmission.In some embodiments of wireless devices according to the presentinvention, modified preambles are used that provide one, two, three orall of the preferable characteristics indicated above.

Combinations of Extensions

Multi-channel extended 802.11 systems might simultaneously transmit onseveral 20 MHz channels, whereas a legacy 802.11 a system only transmitson a single 20 MHz channel using a single antenna, or if the legacysystem does transmit with more than one antenna, each of the antennastransmits the same 802.11a signal, possibly with some delay differencesbetween signals. As a result, data rates can be increased over 802.11adata rates using multiple transmit antennas or multiple channels or acombination of both. Thus, in a communication channel, such as theairspace of a wireless network cloud, several types of packets might bepresent:

1) Legacy SISO (single-input, single-output) 802.11a, 802.11b, or802.11g packets transmitted in a single 20 MHz channel;

2) Extended SISO in multiple 20 MHz channels (e.g., 40, 60, 80, or 100MHz channels)

3) Extended MIMO in a single 20 MHz channel;

4) Extended MIMO in multiple 20 MHz channels (e.g., 40, 60, 80, or 100MHz channels)

Several satisfactory modified preamble structures can be derived by oneof ordinary skill in the art after reading this disclosure. Someexamples are described below. Preferably, the unmodified preamblestructure can provide interoperability and coexistence between SISO andMIMO systems at various channel widths and coexistence between extendedmode systems and legacy systems.

MIMO Single Channel (20 MHz)

A modified preamble can use the same structure as the 802.11a preamble,with a different long training symbol determined from a long trainingsymbol sequence LD. By keeping the same short symbols S and using thesame timing structure as depicted in FIG. 1, a receiver using theextended mode can use the same hardware for detecting the repetitive Sand L symbols, even though the actual contents of the L symbols may bedifferent for the 802.11a extensions.

Various embodiments of wireless devices might use various long trainingsymbol sequences. In one example of a modification, the long trainingsymbol sequence LD has one or more of the following features: 1) it isformulated such that channel estimation can be done for multipletransmitters, 2) it is such that it has a low cross-correlation with theunmodified 802.11a long training symbol sequence, and/or 3) it is usablein a relatively simple process of detecting whether the preamble is an802.11a packet or an extended mode packet, usable in multipath channels.Suitable modified long training symbol sequences are shown as L₂ and L₃,in FIG. 4, but other variations should be apparent upon reading thisdescription.

Channel Estimation

By allowing for channel estimation for multiple transmitters, MIMO orspace-time coding techniques can be supported to achieve 802.11aextensions. One way to do this is by sending a different set ofsubcarriers from each transmitter. As an example, for the case of twotransmitters, a device might modulate its OFDM subcarriers with the 64values of L₃, shown in FIG. 4, where one transmitter transmits the oddsubcarriers {1, 3, . . . , 63} and the other transmitter transmits theeven subcarriers {0, 2, . . . , 62}. Thus, one transmitter would take anIFFT of the odd subcarriers and transmit samples of that time varyingsignal and the other transmitter would take an IFFT of the evensubcarriers and transmit samples of that time varying signal.

L₃ is a modified 802.11a long training symbol sequence, wherein some ofthe subcarriers of the standard 802.11a sequence L₁ (shown in FIG. 2)are inverted, and some subcarriers that are zero in L₁ are non-zero inL₃. The latter has some advantages for channel estimation, but is notnecessary for the purpose of discriminating 802.11a packets fromextended mode packets.

Low Cross-Correlation

The second criterion is that the new training sequence should have a lowcross-correlation with the conventional IEEE 802.11a training sequence.One way to achieve this is to invert every other group of foursubcarriers, which is applied to sequence L₂ to get a new sequence L₃that is nearly orthogonal to both L₁ and L₂. Further, L₃ is constructedsuch that there is also a low cross-correlation between the even and oddelements of L₂ and L₃. These sequences L₂ and L₃ are shown in FIG. 4.The low cross-correlation is illustrated by Equation 1 and Equation 2(note that in Equation 1, a high cross-correlation would have right-handside values closer to −32 or 32, since the sum is not normalized here).$\begin{matrix}{{\sum\limits_{k = 0}^{31}{{L_{2}\left( {2k} \right)}{L_{3}\left( {2k} \right)}}} = {- 1}} & \left( {{Equ}.\quad 1} \right) \\{{\sum\limits_{k = 0}^{31}{{L_{2}\left( {{2k} + 1} \right)}{L_{3}\left( {{2k} + 1} \right)}}} = 0} & \left( {{Equ}.\quad 2} \right)\end{matrix}$

Mode Detection

The low cross-correlation between even and odd elements of L₂ and L₃supports the third criterion, as it makes it possible to detect extendedmode packets by looking at the correlation of L₂ and L₃ with the odd andeven subcarriers of a received packet.

Various methods can be available for a receiver to detect from areceived signal whether a transmitter transmitted a conventional 802.11apacket or an extended mode packet. One method for detecting what type ofpacket was sent will now be described.

In this method, enough of the signal is received to identify what shouldbe the two repeated long training symbols, typically sampled as twoidentical repetitions of 64 samples for each receive antenna. An FFT(fast-Fourier transform) of the sum of the two identical repetitions of64 samples is taken, generating an output sequence s_(i)(k), comprising64 complex values per receive antenna, containing channel amplitudes andphases, as well as phase shifts caused by the long training symbolsequence that was actually used (e.g., sequences such as L₁, L₂, L₃ orL₄).

From the output sequence s_(i)(k), the receiver generates two othersequences, r_(s)(k) and r_(m)(k), by multiplying s_(i)(k) by thesequences L₂ and L₃ for each receive antenna i, as illustrated byEquations 3a and 3b. $\begin{matrix}{{r_{si}(k)} = {\sum\limits_{k = 0}^{63}{{s_{i}(k)}{L_{2}(k)}}}} & \left( {{{Equ}.\quad 3}a} \right) \\{{r_{mi}(k)} = {\sum\limits_{k = 0}^{63}{{s_{i}(k)}{L_{3}(k)}}}} & \left( {{{Equ}.\quad 3}b} \right)\end{matrix}$

Next, the receiver calculates two metrics, m_(m) and m_(s), fromr_(s)(k) and r_(m)(k) using a differential detection operation, such asthat illustrated by Equations 4a and 4b. $\begin{matrix}{m_{s} = {{\sum\limits_{i = 0}^{N - 1}{\sum\limits_{k = 2}^{26}\left\lbrack {{{r_{si}(k)}{r_{si}^{*}\left( {k - 1} \right)}} + {{r_{si}\left( {k + 37} \right)}{r_{si}^{*}\left( {k + 36} \right)}}} \right\rbrack}}}} & \left( {{{Equ}.\quad 4}a} \right) \\{m_{m} = {{\sum\limits_{i = 0}^{N - 1}{\sum\limits_{k = 0}^{11}\left\lbrack {{{r_{mi}\left( {{2k} + 3} \right)}{r_{mi}^{*}\left( {{2k} + 1} \right)}} + {{r_{mi}\left( {{2k} + 41} \right)}{r_{mi}^{*}\left( {{2k} + 39} \right)}} + {{r_{mi}\left( {{2k} + 4} \right)}{r_{mi}^{*}\left( {{2k} + 2} \right)}} + {{r_{mi}\left( {{2k} + 42} \right)}{r_{mi}^{*}\left( {{2k} + 40} \right)}}} \right\rbrack}}}} & \left( {{{Equ}.\quad 4}b} \right)\end{matrix}$

If m_(m)>c*m_(s), then the receiver might assume that the receivedsignal represents a conventional 802.11a packet, otherwise the receiverassumes the packet is an extended mode packet. The constant c ispreferably equal to 1, but may be different.

SISO/MIMO Multiple Channel

Some modified preamble structures described herein provideinteroperability and coexistence between SISO multi-channelpackets/devices and MIMO multi-channel packets/devices, as well ascoexistence between multi-channel packets/devices and legacypackets/devices.

FIG. 5 illustrates the case where out-of-band pilots are attenuated for20 MHz channels used to transmit a MIMO signal. The preamble structurecan be identical to a conventional 802.11a preamble, except that thelong training symbol sequence may use what are otherwise consideredout-of-band subcarriers. Some or all of these out-of-band subcarriersmay also be used in the data symbols to increase the data rate.

In the case of FIG. 5, different channels may be used by differentdevices, but it is also possible that the same device transmits onseveral channels simultaneously. For instance, one device may transmiton channels 1 and 4 simultaneously, while channels 2 and 3 are used byother devices.

If two adjacent channels are used simultaneously by one device, thenthere is no need to attenuate the “out-of-band subcarriers” in themiddle of this 40 MHz band. An example of this is shown in FIG. 6. Theout-of-band subcarriers that are in between the two 20 MHz channels thusneed not be attenuated. In FIG. 4, the sequence L₄ is the long trainingsymbol sequence for a 40 MHz preamble, which contains all 128 subcarriervalues for a 40 MHz channel long training symbol. The first 32 valuesare identical to the last 32 values of a 20 MHz preamble, correspondingto the subcarriers in the left part of a 20 MHz channel. One differencebetween L₄ and two separate 20 MHz long training sequences is that theDC subcarriers are at different locations, so at the position where a 20MHz channel would normally have its DC subcarrier, the 40 MHz sequencecan have a nonzero subcarrier value. In L₄, these are subcarrier numbers33 and 97, respectively.

With unattenuated out-of-band subcarriers, signaling information can becarried on those subcarriers during packet setup, such as signalingoperating and/or extension modes during a preamble, and additional datacan be carried on those subcarriers, to increase the datarate.

FIG. 7 shows the case of four 20 MHz channels.

One example of a modified preamble is the preamble shown in FIG. 1modified as shown in FIG. 8. The long training symbol values for theseout-of-band subcarriers can be the same as in the case of FIG. 1. Thelong training symbol is followed by a replica of the Signal field withidentical subcarrier values in each of the 20 MHz channels. This ensuresthat a receiver that operates on just one of the 20 MHz channels willstill be able to successfully decode at least the first part of thepacket containing the Signal field and defer for the rest of the packet,as decoding the Signal field provides the receiver with informationabout the length of the packet and thus how long to defer. The sametechnique can be extended to an arbitrary number of channels.

FIG. 8 shows a preamble for a two transmitter MIMO packet. The structureis the same as for 802.11a, but some differences are that a) l₀, l₁, d₀,d₁ may contain out-of-band subcarriers, b) s₁, l₁, d₁ can be cyclicallyshifted relative to s₀, l₀, d₀ or c) l₀ and l₁ can contain subcarriersequences that have a low cross-correlation with the same subcarriersequences of the 802.11a long training symbol sequence.

Interoperability

Interoperability between the different extended modes can be ensured bytransmitting the same preamble and signal field in each 20 MHz channel.The preamble time structure can be the same as that of IEEE 802.11a, asillustrated in FIG. 1. For a 20 MHz MIMO transmitter, the long trainingsymbol L can be modified to facilitate MIMO channel estimation andinclude out-of-band pilots. In one example of an extended transmitterusing a plurality of channels, the transmitter transmits an identicalcopy of the preamble and signal field in each 20 MHz channel used bythat transmitter where the out-of-band pilots only have to be attenuatedat the edges of a multi-channel and not between adjacent channels of themulti-channel. The out-of-band subcarriers of the signal field in mightcontain different data bits for different 20 MHz channels, to signalinformation such as the transmitter's multi-band mode, MIMO mode,channel number, data rate, and/or coding rate.

By transmitting the same preamble and signal field in any 20 MHzchannel, it is ensured that an extended device that only demodulates one20 MHz channel at least is able to decode the signal field. From theinformation in the signal field, the single-channel extended device caneither properly defer for the duration of the packet or find out whatextended mode is used for this packet in the case that this informationis encoded in the signal field. For instance, the receiver could detectfrom the signal field that the packet is transmitted over four adjacentchannels, after which the extended receiver can decide to switch to afour-channel receiving mode.

Notice that it typically does not matter for a single-channel 20 MHzreceiver whether the out-of-band subcarriers depicted in FIGS. 5-7 areattenuated. For instance, if a single-channel receiver demodulateschannel 2 out of the 4 transmitted channels shown in FIG. 7, the receivefilter of that single-channel receiver will partly attenuate theout-of-band subcarriers as well as suppress the adjacent channels 1 and3 to the point where these adjacent channels do not cause interferenceto the desired signal of channel 2.

Coexistence

One method of having coexistence between extended devices and legacyIEEE 802.11a and IEEE 802.11g devices is by keeping the preamblestructure in each 20 MHz channel the same as for IEEE 802.11a. IEEE802.11a specifies an energy detect based defer behavior, which providessome level of coexistence. However, to guarantee that legacy devicesproperly defer for all extended mode packets down to received powerlevels of −82 dBm or other suitable levels, the receivers have to beable to successfully decode the signal field, which contains the lengthinformation of the packet.

Some ways to do this are described by Bangerter, B., et al.,“High-Throughput Wireless LAN Air Interface”, Intel Technology Journal,Vol. 7, Issue 3 (August 2003) (hereinafter “Bangerter”) and Boer, J., etal., “Backwards Compatibility”, IEEE 802.11 presentation, DocumentNumber 802.11-03/714rO (September 2003) (hereinafter “Boer”).

Bangerter describes the use of multiple 802.11a preambles spread infrequency such that 20 MHz channel legacy 802.11a devices will defer formultiple channel devices, but additional advantages can be had throughthe use of out-of-band pilots or MIMO preambles, as described elsewhereherein.

Boer describes some possible MIMO preambles having some limitedbenefits. In one method described in Boer, each MIMO transmittertransmits an 802.11a preamble while the other transmitters transmitnothing. While this makes distinguishing easier, training issignificantly longer and that reduces throughput. In another methoddescribed in Boer, each MIMO transmitter transmits a part of the 802.11asubcarriers. For example, for two transmitters, one transmittertransmits all odd subcarriers and the other transmitter transmits alleven subcarriers. However, without more, mode detection based on thetraining symbols might not be possible with that technique.

A novel way of enabling coexistence or furthering coexistence for MIMOpackets is to apply a cyclic delay shift on the long training symbol andSignal field IFFT outputs prior to applying the guard time extension.For example, assume L(k) and D(k) are the 64 subcarrier values for thelong training symbol and Signal field symbol, respectively. For aconventional 802.11a single transmitter transmission, the time samplesfor the long training symbol are derived by taking the 64-point IFFT ofL(k) to obtain l(i) and transmitting the samples of l(i). Thus, with theguard time, the long training symbol and guard time are constructed as[l(33:64) l (1:64) l (1:64)], i.e., the IFFT output is repeated twiceand the last 32 samples are prepended to form the long training guardinterval. As with the conventional timing, the long training guardinterval (32 samples) is twice as long as the guard interval for 802.11adata symbols (16 samples). The signal field is formed by [d(49:64)d(1:64)], where d(1:64) are the 64 samples of the IFFT of D(k).

In the case of a two transmitter MIMO device, the first transmitterwould transmit the long training symbol and signal field like that of802.11a. The second transmitter would apply a cyclic shift such thatinstead of the IFFT output l(1:64), it uses the cyclically shiftedsamples ls=[l(33:64) l(1:32)] to construct the long training symbolsamples [ls(33:64) ls(1:64) ls(1:64)]. For the signal field, it uses theshifted samples ds=[d(33:64) d(1:32)] to construct the signal field as[ds(49:64) ds(1:64)].

In a legacy 802.11a packet, one 3.2 microsecond repetition of the longtraining symbol L as shown in FIG. 1 is expressed in the time domain asthe IFFT of L(k), where L(k) contains 64 subcarrier values, of which 52are non-zero. The time samples l(i) are given as shown in Equation 5,where the subcarrier values of L(k): $\begin{matrix}{{l(i)} = {\sum\limits_{k = 0}^{63}{{L(k)}{\exp\left( {j\quad\frac{2{\pi\mathbb{i}}\quad k}{64}} \right)}}}} & \left( {{Equ}.\quad 5} \right)\end{matrix}$

In the extended modes described herein, some possible modifications willbe described. First, L(k) can contain more than 52 non-zero subcarriers.Second, in the case of MIMO transmission, l(i) can have a cyclic shiftthat may be different for each transmitter. The shifted signal l_(k)(i)can be derived from l(i) as l_(k)(i)=l([i+64-dk]%64), where “%” denotesthe modulo operator and dk is the cyclic delay of transmitter k in 20MHz samples. This expression assumes a 20 MHz sampling rate, such thatthere are 64 samples in a 3.2 microsecond interval. An alternativemethod of generating the cyclic shift is to apply a phase ramp rotationto all subcarrier values of L(k) prior to calculating the IFFT, such asthat shown by the example of Equation 6. $\begin{matrix}{{l_{k}(i)} = {\sum\limits_{k = 0}^{63}{{L(k)}{\exp\left( {{- j}\quad\frac{2\pi\quad{kd}_{k}}{64}} \right)}{\exp\left( {j\frac{\quad{2{\pi\mathbb{i}}\quad k}}{64}} \right)}}}} & \left( {{Equ}.\quad 6} \right)\end{matrix}$

A MIMO transmitter can have two or more transmit antennas (or antennaarrays, as the case may be). For a MIMO system with two transmitantennas and two different transmit data streams, preferred values forthe cyclic delay values d_(k) are 0 and 32 samples, respectively. Thiscorresponds to a cyclic delay of 1.6 microseconds between the twotransmitters. For three transmitters, d_(k) can be 0, 22, and 43samples, respectively. For four transmitters, d_(k) can be 0, 16, 32,and 48 samples, respectively.

At the receiver side, the channel estimates for each transmitter signalcan be estimated by a process such as that shown in FIG. 9. As shownthere, the process begins with receiving signals and sampling for thelong training symbol (step S1). Then, a 64-point FFT of the receivedlong training symbol samples is done (step S2), as is done forconventional 802.11a preamble reception. Next, each subcarrier ismultiplied by known pilot values (step S3), and an IFFT of the result istaken to get a 64-point impulse response estimate (step S4).

In the case of a MIMO transmission, these 64 samples contain the cyclicshifted impulse responses of all different transmitters. With that, thereceiver can isolate the impulse responses for each MIMO transmitter(step S5). For MIMO with two transmit streams, this can be done byseparating the first 32 samples and last 32 samples. For four transmitstreams, groups of 16 samples can be extracted.

From the extracted impulse responses per transmitter, channel estimatescan be derived (step S6) for all subcarriers by taking a 64-point FFT ofeach impulse response, where the sample values are appended by zerovalues to get 64 input values.

Signaling Extended Modes

There are several different ways to signal what mode is used:

1) Beaconing: Each access point regularly transmits beacons. Bytransmitting these beacons using a legacy 802.11a rate on all 20 MHzchannels used by the access point, it can be ensured that any device canreceive these beacons. The beacon can contain information about whichchannels are used simultaneously and what extended modes are supported,so each extended device can adjust its mode accordingly.

2) Multiple transmitter detection: If a special long training symbol isused for MIMO transmitters such as the cyclic shifted symbol describedherein or other techniques described herein, then these special longtraining properties can be used to detect whether a packet is a MIMOpacket. For instance, if the cyclic shifted long training symbol is usedwith two MIMO transmitters, the receiver can detect this by checking ifthe combined impulse response (obtained from step S4 above) contains twodistinct impulse responses separated by 32 samples.

3) Signal field: The reserved bit of the Signal field can be used tosignal the use of MIMO. It is also possible to extend the Signal fieldby transmitting an extra symbol. An example of this is shown in Boer.There is a reserved bit in the Signal field that is always zero for802.11a devices but could be set to 1 to signal MIMO packets. It is alsopossible to send an extra signal field symbol after the normal 802.11asymbol to signal MIMO rates.

4) Out-of-band pilots: The out-of-band pilots of the long trainingsymbol that are not present in 802.11a can be used to signal differentmodes. For example, subcarriers 28 through 38 in L₁ are zero, but theymight be set (as with L₂, L₃, etc.) to some arbitrary but known values.The receiver can use the presence of these subcarriers as a way todetect MIMO modes and the particular pattern of presence to detect amongseveral modes.

5) Out-of-band subcarriers in the Signal field: Extra subcarriers can beused to signal different extended modes. The use of extra subcarriershas advantages in that a) it does not cost extra preamble overhead, andb) a legacy 802.11a device ignores the out-of-band subcarriers.

1. A method of transmitting signals using a plurality of transmitantennas, the method comprising: allocating the data to be transmittedamong the plurality of transmit antennas, wherein at least one of theplurality of transmit antennas transmits some data that is nottransmitted by all of the other of the plurality of transmit antennas;transmitting a modified preamble from each of the plurality of transmitantennas, wherein the modified preamble is distinguishable at a receiverfrom a conventional 802.11a preamble.
 2. The method of claim 1, whereinthe plurality of transmitters transmit data in total at an extended rateabove a corresponding 802.11a data rate.
 3. The method of claim 1,wherein the modified preamble comprises a modified long training patterndistinct from a conventional 802.11a long training pattern.
 4. Themethod of claim 3, wherein at least a part of the modified long trainingpattern has a low cross correlation with a corresponding part of theconventional 802.11a pattern, thereby facilitating discrimination basedon cross correlation.
 5. The method of claim 4, wherein the at least apart of the modified long training pattern is transmitted using morethan one of the plurality of transmit antennas such that it isreceivable and processable by one or more receivers.
 6. A method ofdiscriminating between a packet sent with a conventional 802.11a rate orwith an extended rate, comprising: receiving one or more signals fromone or more transmitters, the one or more signals including a longtraining subcarrier; multiplying the long training subcarrier with aconventional 802.11a long training pattern to form a first product;multiplying the long training subcarrier with an extended 802.11a longtraining pattern to form a second product; determining, from the firstproduct and the second product, which long training pattern was morelikely to have been sent for the received long training subcarrier; anddiscriminating as to which type of packet was sent based on the morelikely sent long training subcarrier.
 7. A method of transmittingsignals using a plurality of transmit channels, the method comprising:allocating the data to be transmitted among the plurality of transmitchannels, wherein at least one of the plurality of transmit channelstransports some data that is not transmitted over all of the other ofthe plurality of transmit channels; transmitting a modified preamblefrom each of the plurality of transmit channels, wherein the modifiedpreamble is distinguishable at a receiver from a conventional 802.11apreamble and includes an out-of-band component.
 8. The method of claim7, wherein the plurality of transmit channels comprise a plurality offrequency channels.
 9. The method of claim 8, wherein the plurality offrequency channels are adjacent 20 MHz channels.
 10. A method oftransmitting signals using a plurality of transmit channels, the methodcomprising: allocating the data to be transmitted among the plurality oftransmit channels, wherein at least one of the plurality of transmitchannels transports some data that is not transmitted over all of theother of the plurality of transmit channels; for at least one set of atleast two adjacent transmit channels, transmitting data over the setwherein at least some data is encoded in out-of-band subcarriers atfrequencies between frequencies allocated to the at least two adjacenttransmit channels.
 11. In a communications system having a channeldivided into a plurality of adjacent frequency bands separated byout-of-band frequency ranges, wherein data is transmitted within thebands of the plurality of frequency bands, a method of increasing datacapacity of the channel comprising: for data to be transmitted from atransmitter, allocating a first portion of the data among the pluralityof transmit frequency bands and allocating a second portion of the datato at least one out-of-band frequency range when the first portion isallocated to adjacent bands, wherein the at least one out-of-bandfrequency range includes an out-of-band frequency range between theadjacent bands; transmitting the first portion within the plurality oftransmit frequency bands; and transmitting the second portion within theat least one out-of-band frequency range.
 12. The method of claim 11,further comprising: prior to transmitting at least the second portion ofthe data, transmitting one or more training symbols usable for areceiver to estimate transmission characteristics of the out-of-bandfrequency ranges; and using received signal of the one or more trainingsymbols to modify processing of a received signal corresponding to thesecond portion of the data to account for the transmissioncharacteristics of the out-of-band frequency ranges.
 13. A method ofdiscriminating between a packet sent as a conventional 802.11a packetand a packet sent using an extended mode not normally supported underthe conventional 802.11a standard, the method comprising: receiving asignal from a wireless medium, wherein the signal was transmitted froman extended mode transmitter as a packet wherein packet data is precededby a packet preamble and wherein the packet preamble is generated from acyclically shifted 802.11a preamble; demodulating the signal to obtain ademodulated signal; decoding, from the demodulated signal, a packet datasequence including a cyclically shifted 802.11a preamble when receivingpacket data from an extended mode transmitter and a conventional 802.11apreamble when receiving packet data from a conventional 802.11atransmitter; and discriminating as to which type of packet was sentbased on the received packet data sequence.
 14. The method of claim 13,wherein the extended mode includes at least a MIMO extended mode whereinthe packet preamble is generated from the cyclically shifted 802.11apreamble.
 15. The method of claim 14, further comprising performing MIMOchannel estimation using the received preamble data.
 16. The method ofclaim 13, wherein the further comprising performing MIMO channelestimation using the received preamble data.
 17. The method of claim 13,wherein the signal transmitted from an extended mode transmitter is suchthat legacy devices can decode a signal field of the preamble.
 18. Themethod of claim 13, further comprising detecting that the signaltransmitted used from an extended mode transmitter using a MIMO mode,the detecting using at least one out-of-band subcarrier
 19. The methodof claim 13, further comprising detecting that the signal transmittedused from an extended mode transmitter using a MIMO mode, the detectingincluding detecting a presence of cyclically shifted preamblecomponents.
 20. A method of transmitting a packet, using a MIMOtransmitter having a plurality of antennas, over a wireless networkwherein receivers operating as conventional 802.11a receivers might bepresent, the method comprising: obtaining data fields of a packet to betransmitted; generating preamble fields of the packet to be transmitted,including an extended mode preamble distinguishable at a receiver from aconventional 802.11a preamble, wherein a conventional 802.11a receivercan decode one or more fields of the extended mode preamble; andtransmitting the packet including the extended mode preamble.
 21. Themethod of claim 20, wherein the fields of the extended mode preambleinclude a modified short training sequence.
 22. The method of claim 20,wherein the fields of the extended code preamble include a modified longtraining sequence.
 23. The method of claim 20, wherein the fields of theextended mode preamble include a modified signal field.
 24. A method ofcommunicating a packet, using a MIMO transmitter having a plurality ofantennas, over a wireless medium to a MIMO receiver wherein receiversoperating as conventional 802.11a receivers might be listening totransmissions in the wireless medium, the method comprising: obtainingdata fields of a packet to be transmitted; generating preamble fields ofthe packet to be transmitted, including an extended mode preamble;transmitting the packet, including the extended mode preamble, as asignal into the wireless medium; receiving a representation of thesignal from a wireless medium; at a receiver, demodulating the signal toobtain a demodulated signal; at the receiver, decoding, from thedemodulated signal, a packet data sequence including data representingat least a portion of a preamble; where the receiver is a MIMO receiver,processing the packet data sequence according to an extended modeoperation; and where the receiver is a conventional 802.11a receiver,processing the packet data sequence to determine at least one validconventional 802.11a preamble field and deferring further data receptionrelated to that packet data sequence after determining, from thepreamble, that the packet data sequence represents a packet not inconformance with a conventional 802.11a packet.
 25. The method of claim24, wherein the fields of the extended mode preamble include a modifiedshort training sequence.
 26. The method of claim 24, wherein the fieldsof the extended mode preamble include a modified long training sequence.27. The method of claim 24, wherein the fields of the extended modepreamble include a modified signal field.
 28. A method of transmittingsignals using a plurality of transmit channels, the method comprising:allocating the data to be transmitted among the plurality of transmitchannels, wherein at least one of the plurality of transmit channelstransports some data that is not transmitted over all of the other ofthe plurality of transmit channels; transmitting a modified preamblefrom each of the plurality of transmit channels, wherein the modifiedpreamble is usable for performing channel estimation and at least afirst part of the modified preamble for at least a first of theplurality of transmit channels is a cyclically shifted version of asecond part of the modified preamble for at least a second of theplurality of transmit channels.
 29. The method of claim 28, wherein thefirst part and the second part comprise signal sequences with a lowcross-correlation between long training symbols.
 30. The method of claim28, wherein the first part and the second part comprise signal sequenceswith a low cross-correlation between long training symbols.
 31. Themethod of claim 28, further comprising MIMO synchronization.
 32. Themethod of claim 28, wherein the data to be transmitted is allocated to aplurality of subcarriers, the subcarriers of the plurality ofsubcarriers are allocated among transmit channels, and each transmitchannel is associated with a distinct antenna.
 33. The method of claim28, wherein the data to be transmitted is allocated to a plurality ofsubcarriers and some of the subcarriers of the plurality of subcarriersare inverted relative to other subcarriers of the plurality ofsubcarriers
 34. The method of claim 28, wherein the data to betransmitted is allocated to a plurality of subcarriers inlcuding atleast one out-of-band subcarrier.
 35. The method of claim 28, furthercomprising estimating channel response by: receiving signals andsampling for a long training symbol; computing a 64-point FFT of thereceived long training symbol; multiplying each subcarrier is multipliedby known pilot values; computing an IFFT of the result of themultiplication, resulting in a 64-point impulse response estimate;isolating each of a plurality of impulse responses, one per MIMOtransmitter; and deriving channel estimates for all subcarriers from theisolated impulse responses by taking a 64-point FFT of each of theplurality of impulse responses, where the sample values are appended byzero values to get 64 input values as needed.